1. Field
This invention relates to X-ray apparatus which are capable of switching emission levels and more particularly to an X-ray tube system which can rapidly switch emission levels so that the tube can be use for imaging, fluorescence, and other applications using differing energies.
2. Background of the Invention
In the Homeland Security industry, dual energy X-ray is used for screening of carry-on baggage, screening of checked baggage, and screening of cargo. The use of Dual Energy X-ray for Homeland Security allows users to distinguishing different materials, aiding in the detection of contraband, explosives, and illicit material as well as in the identification of materials in the object being scanned.
Dual energy X-ray is used in the medical industry for dual energy computed tomography (CT), dual energy X-ray absorptiometry, dual energy imaging, and other applications. Besides the Homeland Security and medical markets, dual energy X-ray is used in the non-destructive testing, dental, food packaging, coal, and other industries.
There are several methods of producing dual energy X-rays and conducting dual energy analysis. In the simplest form, two or more detectors are used, one behind the other, and a filter is placed between the detectors. One detector receives one energy range, and the other detector receives a second energy range due to the filter. Alternately, an energy discriminating detector can be used to separate energies, such as a CZT detector. Another approach commonly used is to vary the end output energy of the X-ray source, so that the X-ray source emits two or more X-ray energy ranges. Yet another approach is to use more than one X-ray tube, and each X-ray tube emits one or more energy ranges.
Desirable characteristics for a dual energy source are fast switching speeds between energies, high stability, small form factor, simplicity, lowest system cost, long life, high reliability, and the ability to rapidly change the flux for each separate energy. Having the same focal spot location is also highly desirable for some applications. The exact combination of characteristics that is the most valuable depends on the application.
Several approaches have been developed or proposed that have some but not all of these characteristics.
One approach involves using two or more separate X-ray tubes, and running each source independently. This provides rapid switching speeds. The problem with this approach is that it takes up the space of two X-ray sources, and each source has a different focal spot location. Thus the solution is relatively large, and produces two different focal spot locations. In imaging applications, complexity, alignment and misregistration between the images is another issue with using two separate sources.
An alternate solution detailed in U.S. Pat. No. 5,661,774, is to use electronics that rapidly switch energies of the X-ray tube. An application of this would be in dual energy CT in medical and Homeland Security application where rapid switching is desirable. An issue with this approach is the difficulty in switching the X-ray energy at high speed. This requires complex electronics, drives up system costs and is not optimum for tube life. Another issue with this approach is that the rapid switching induces variation in the output energy of the X-ray source.
U.S. Pat. No. 6,188,747 details an x-ray source that uses two independent anodes, and two cathodes. Thus it has two x-ray sources in one X-ray tube, resulting in increased cost, complexity, weight, and shielding. The limitation of this approach is the physical size of the unit, and the spot size location being different for each anode.
An approach that solves the spot location problem is to use two cathodes and one anode in the X-ray source, and to physically separate the cathodes. This is detailed in U.S. Pat. Nos. 7,529,344 and 7,792,241 and is illustrated in FIG. 17. The cathodes 1 are at different energies, and the cathodes 1 are separated, so the cathodes 1 do not arc. With this solution, each cathode 1 has a separate grid 3. A limitation with this approach is the difficulty in alignment of the cathode 1 and grids 3 and even more difficult, alignment of the 2 spots. Yet another issue is the physical size of the resulting X-ray tube.